Why does the Greek lowercase o look disproportionately bold?

Question

I'm using unicode-math and I've decided I really like the Latin Modern Math Greek letters better than those from other fonts. Unless it's an illusion, the omicron (o) always seems to look disproportionately bold, even when not explicitly bolded. Is there a particular reason for this? I would otherwise use the Greek letters from TeX Gyre Schola Math but there doesn't seem to be a \symbfsfup variant, and there too the omicron looks disproportionately bold.

MWE:

% !TEX TS-program = lualatexmk
% !TEX encoding = UTF-8 Unicode

\documentclass{article}
\RequirePackage[math-style=ISO]{unicode-math}
\setmathfont{TeX Gyre DejaVu Math}
\setmathfont[range=it/{greek,Greek}]{Latin Modern Math}
\setmathfont[range=bfit/{greek,Greek}]{Latin Modern Math}
\setmathfont[range=up/{greek,Greek}]{Latin Modern Math}
\setmathfont[range=bfup/{greek,Greek}]{Latin Modern Math}
\setmathfont[range=bfsfup/{greek,Greek}]{Latin Modern Math}

\begin{document}
\[ 
  \alpha\beta\gamma\delta\epsilon\varepsilon\zeta\eta\theta\vartheta\iota\kappa
  \lambda\mu\nu\xi o\pi\varpi\rho\varrho\sigma\varsigma\tau\upsilon\phi\varphi
  \chi\psi\omega\Delta\Gamma\Theta\Lambda\Xi\Pi\Sigma\Upsilon\Phi\Psi\Omega 
\]
\[ \symbf{% 
  \alpha\beta\gamma\delta\epsilon\varepsilon\zeta\eta\theta\vartheta\iota\kappa
  \lambda\mu\nu\xi o\pi\varpi\rho\varrho\sigma\varsigma\tau\upsilon\phi\varphi
  \chi\psi\omega\Delta\Gamma\Theta\Lambda\Xi\Pi\Sigma\Upsilon\Phi\Psi\Omega}
\]
\[ \symup{% 
  \alpha\beta\gamma\delta\epsilon\varepsilon\zeta\eta\theta\vartheta\iota\kappa
  \lambda\mu\nu\xi o\pi\varpi\rho\varrho\sigma\varsigma\tau\upsilon\phi\varphi
  \chi\psi\omega\Delta\Gamma\Theta\Lambda\Xi\Pi\Sigma\Upsilon\Phi\Psi\Omega}
\]
\[ \symbfup{% 
  \alpha\beta\gamma\delta\epsilon\varepsilon\zeta\eta\theta\vartheta\iota\kappa
  \lambda\mu\nu\xi o\pi\varpi\rho\varrho\sigma\varsigma\tau\upsilon\phi\varphi
  \chi\psi\omega\Delta\Gamma\Theta\Lambda\Xi\Pi\Sigma\Upsilon\Phi\Psi\Omega}
\]
\[ \symbfsfup{% 
  \alpha\beta\gamma\delta\epsilon\varepsilon\zeta\eta\theta\vartheta\iota\kappa
  \lambda\mu\nu\xi o\pi\varpi\rho\varrho\sigma\varsigma\tau\upsilon\phi\varphi
  \chi\psi\omega\Delta\Gamma\Theta\Lambda\Xi\Pi\Sigma\Upsilon\Phi\Psi\Omega}
\]
\end{document}

UPDATE: Here is the MWE using \omicron and the resulting output. Newcomers to LaTeX who use the legacy engines (like pdflatex) may not know that the unicode-math package defines \omicron. Legacy TeX/LaTeX books explicitly point out there is no such command and readers may not know about the added functionality of modern engines (e.g. lualatex) and the power of Unicode.

% !TEX TS-program = lualatexmk
% !TEX encoding = UTF-8 Unicode

\documentclass{article}
\RequirePackage[math-style=ISO]{unicode-math}
\setmathfont{TeX Gyre DejaVu Math}
\setmathfont[range=it/{greek,Greek}]{Latin Modern Math}
\setmathfont[range=bfit/{greek,Greek}]{Latin Modern Math}
\setmathfont[range=up/{greek,Greek}]{Latin Modern Math}
\setmathfont[range=bfup/{greek,Greek}]{Latin Modern Math}
\setmathfont[range=bfsfup/{greek,Greek}]{Latin Modern Math}

\begin{document}
\[ 
  \alpha\beta\gamma\delta\epsilon\varepsilon\zeta\eta\theta\vartheta\iota\kappa
  \lambda\mu\nu\xi\omicron\pi\varpi\rho\varrho\sigma\varsigma\tau\upsilon\phi\varphi
  \chi\psi\omega\Delta\Gamma\Theta\Lambda\Xi\Pi\Sigma\Upsilon\Phi\Psi\Omega 
\]
\[ \symbf{% 
  \alpha\beta\gamma\delta\epsilon\varepsilon\zeta\eta\theta\vartheta\iota\kappa
  \lambda\mu\nu\xi\omicron\pi\varpi\rho\varrho\sigma\varsigma\tau\upsilon\phi\varphi
  \chi\psi\omega\Delta\Gamma\Theta\Lambda\Xi\Pi\Sigma\Upsilon\Phi\Psi\Omega}
\]
\[ \symup{% 
  \alpha\beta\gamma\delta\epsilon\varepsilon\zeta\eta\theta\vartheta\iota\kappa
  \lambda\mu\nu\xi\omicron\pi\varpi\rho\varrho\sigma\varsigma\tau\upsilon\phi\varphi
  \chi\psi\omega\Delta\Gamma\Theta\Lambda\Xi\Pi\Sigma\Upsilon\Phi\Psi\Omega}
\]
\[ \symbfup{% 
  \alpha\beta\gamma\delta\epsilon\varepsilon\zeta\eta\theta\vartheta\iota\kappa
  \lambda\mu\nu\xi\omicron\pi\varpi\rho\varrho\sigma\varsigma\tau\upsilon\phi\varphi
  \chi\psi\omega\Delta\Gamma\Theta\Lambda\Xi\Pi\Sigma\Upsilon\Phi\Psi\Omega}
\]
\[ \symbfsfup{% 
  \alpha\beta\gamma\delta\epsilon\varepsilon\zeta\eta\theta\vartheta\iota\kappa
  \lambda\mu\nu\xi\omicron\pi\varpi\rho\varrho\sigma\varsigma\tau\upsilon\phi\varphi
  \chi\psi\omega\Delta\Gamma\Theta\Lambda\Xi\Pi\Sigma\Upsilon\Phi\Psi\Omega}
\]
\end{document}

Answer 1

You used a latin o not an omicron (\omicron) which is particularly noticeable here as it means you get the dejavu math character not latin modern.

\omicron is defined in unicode-math. It did not exist in classic TeX, which was limited to 127-character 7-bit encoding, so omitted definitions for anything that looked the same in both Latin and Greek.